1. Field of the Invention
The present invention relates to a valve lifter coming into contact with a cam for operating a valve of an engine, and more particularly, to an aluminum alloy valve lifter and a method of producing the same.
2. Description of the Related Art
Valve lifters used in an engine of an automobile or the like are usually made of a light alloy, such as an aluminum alloy, instead of steel, to reduce weight and improve the fuel consumption efficiency (see, for example, Japanese Examined Patent Publication (JP-B) No. 47-50885). Use of the aluminum alloy valve lifter, however, causes problems with the wear resistance of the lifter when in contact with a cam for operating a valve, an end of a valve rod (stem), and an inside surface of a guide hole therefor formed in an aluminum cylinder head. Various proposals have been made for light metal (aluminum alloy) valve lifters having a cylindrical portion and a disc portion formed within a cylindrical portion. For example, as disclosed in Japanese Unexamined Patent Publication (JP-A) No. 58-165508, a wear-resistant metal part coming into contact with the cam and the valve rod is inserted in an cast aluminum alloy valve lifter; and a peripheral surface thereof is plated with Fe-P. In this case, the wear-resistant metal part serves as an inner shim, and as an adjusting shim, but this makes it impossible to replace the adjusting shim only, and thus makes it difficult to carry out maintenance of the valve lifter. In the valve lifter disclosed in Japanese Unexamined Patent Publication (JP-A) No. 58-165508, a projecting part of a wear-resistant material is inserted in a cast aluminum alloy lifter body inside the disc portion of the body, and a removable adjusting shim is lifted to the disc portion. In this case, however, the top surface of the disc portion of the valve lifter body does not have sufficient wear-resistance to carry the adjusting shim, and the wear-resistance of the peripheral surface of the lifter body sliding in a guide hole of a cylinder head, also is insufficient.
A valve lifter disclosed in Japanese Unexamined Patent Publication (JP-A) No. 58-214609 is formed of a light alloy casting (e.g., aluminum alloy casting). A top surface for contact with a cam, a back surface for contact with a valve rod and a disc portion of the valve lifter are coated with a hard material by thermal spraying. However, it is difficult to accurately form the sprayed coating of a hard material on the back surface of the disc portion of the valve lifter, and the peripheral surface of the lifter sliding in the guide hole still does not have a sufficient wear resistance.
In an aluminum alloy valve lifter disclosed in Japanese Examined Patent Publication (JP-B) No. 47-50885 a filler for contact with the cam is sprayed into a recess; in a disc portion, the recess has a profile such that the filler will not separate therefrom. To form a reverse tapered portion of the recess prior to the thermal spraying, however, a complicated cutting process must be carried out on the disc portion, and it is difficult to apply a desired sprayed coating onto the reverse tapered portion. Furthermore, wear resistance of the peripheral surface of the valve lifter sliding in the guide hole is still insufficient.
According to JIII (Japan Institute of Invention and Innovation) Journal of Technical Disclosure No. 85-15251, a valve lifter body can be made of titanium and the entire surface thereof subjected to a nitriding process to increase the durability thereof, but the use of titanium greatly increases the costs.
In a well known surface treatment of a sliding part of a light alloy such as aluminum alloy, a wear-resistant sprayed coating (layer) is formed on the sliding part by thermally spraying a coating of a ferrous metal thereon by an electric arc spraying or plasma spraying process. The plasma spraying process is disclosed, for example, in Japanese Unexamined Patent Publication (JP-A) No. 53-6238 and 53-42148 and Japanese Examined Patent Publication (JP-B) No. 57-34346. When the thermal spraying process is used to apply a surface treatment of a peripheral surface of an aluminum alloy valve lifter, a sprayed coating having a good wear resistance can be formed uniformly over the entire peripheral surface.
In the plasma spraying process, an electric arc is generated between a cathode and a copper nozzle anode, and a working gas (Ar, Ar+H.sub.2 or Ar+N.sub.2) is made to flow spirally through the nozzle and is heated by the arc, so that a plasma jet having a high temperature and a high speed is spouted from the nozzle. The material (powder) to be sprayed is fed into the plasma jet, melted, and impinged on a workpiece surface to form a sprayed coating thereon. In the electric arc spraying process, two wires formed of the material to be sprayed are fed continuously. An electric arc is generated between the ends of the two wires and the wires are melted by the arc. The melt is formed into molten particles by an air jet spout blown from behind, and the molten particles are impinged on a workpiece surface to form a sprayed coating thereon.
The sprayed coating formed by the arc spraying process has a greater porosity than that of the sprayed coating formed by the plasma spraying process. This is because the molten particles obtained from the wires in the arc spraying process are larger than the particles obtained from the powder in the plasma spraying process, and the speed of travel of the molten particles in the arc spraying process is slower than that of the particles in the plasma spraying process. Therefore, particles adhering to the workpiece surface in the arc spraying process are not crushed to the degree occurring in the plasma spraying process, and thus pores between the particles adhering in the arc spraying process are larger than those of the plasma spraying process. Where the porosity is great (i.e., a large number of pores exist), the wear resistance and peeling resistance of the sprayed coating are lowered. The above also applies to the formation of an Fe-C type sprayed coating on a peripheral surface of an aluminum valve lifter, by the arc spraying process. In this connection, if the hardness of the sprayed coating is too low, the wear resistance thereof is lowered. Conversely, if the hardness of the sprayed coating is too high, the coating causes a high degree of wear of the surfaces with which it is in contact, for example, the surface of a guide hole.
When the aluminum alloy valve lifter is used, wear of the upper portion, including the top end and the lower portion, and including the bottom end of the valve lifter, is greater than that of the center portion, due to sliding kinetic behavior. Namely, wear caused by the sliding contact of the peripheral surface of the valve lifter and the inner surface of the guide hole appears on the upper and lower portions having a width of from 5 to 7 mm from the top and bottom end, respectively, of the peripheral surface, but does not appear at the center portion thereof. Nevertheless, disregarding the above wear phenomena, the sprayed coating on the peripheral surface is generally uniform.
Furthermore, the ferrous metal sprayed coating formed on the peripheral surface of the aluminum alloy valve lifter is ground by a centerless grinder, to finish the valve lifter to the required dimensions and to prevent the sprayed coating from causing wear of the inner surface of the guide hole. In the centerless grinding, a water-soluble oil (grinding fluid) is used to prevent an undue load on the grinding wheel; and if the valve lifter is ground after the coating treatment, and water-soluble oil adheres to, and penetrates the ferrous metal sprayed coating and generates rust (or corrosion), which causes wear of both the sprayed coating and the inner surface of the guide hole. Therefore, after the grinding step the ground surface of the lifter is coated with a rust-preventing oil or the like by dipping or spraying.
Nevertheless of although coated with a rust-preventing oil, water held in pores of the sprayed coating cannot be completely removed and thus rust may be generated during a long period of storage in atmospheric conditions before installation thereof into an engine.